A supply modulator, a modulated power supply circuit, and associated control method are provided. The modulated power supply circuit includes the supply modulator and a DC-DC voltage converter, and the supply modulator includes a linear amplifier and a switching converter. The linear amplifier generates an AC component of a modulated voltage according to a regulated voltage and an envelope tracking signal. The supply voltage is converted to the regulated voltage by the DC-DC voltage converter, and the regulated voltage is greater than or less than the supply voltage. The switching converter includes a step-down circuit and a path selection circuit. The path selection circuit selects one of the supply voltage and the regulated voltage as a DC input voltage. The step-down circuit converts the DC input voltage to a DC component of the modulated voltage which is less than the DC input voltage.